In recent years, polymer electrolyte fuel cells have been drawing attention as a power source for electric vehicles. Polymer electrolyte fuel cells (PEFC) have been adapted in practical use for various applications as they generate electricity at ordinary temperature.
Generally, a fuel cell system is divided into the cathode and the anode by a solid polymer electrolyte membrane, and oxygen in the air and hydrogen in a fuel gas are supplied to the cathode and the anode, respectively, so that oxygen and hydrogen are chemically reacted to generate electricity for driving an outer load.
As one important parameter which affects power generation efficiency in the fuel cell system, the ionic conductivity of hydrogen ions migrating in the solid polymer electrolyte membrane is considered. The higher the ionic conductivity, the more the number of hydrogen ions that can migrate in the solid polymer electrolyte membrane per unit of time increases. Therefore, the amount of electricity generated by the electrochemical reaction will increase proportionally.
However, in order to maintain a high ionic conductivity, it is required that moisture be applied to the solid polymer electrolyte membrane at all times to prevent the membrane from drying. For this reason, the fuel cell system must include a humidifier.
As a humidifier used in this purpose, a supersonic humidifier, a nozzle injection humidifier, a steam humidifier, and the like may be employed. However, in terms of less electric power consumption and less installation space, a humidifier utilizing hollow fiber membranes is generally used.
One example of a humidifying system for a fuel cell equipped with a humidifier utilizing hollow fiber membranes is illustrated in FIG. 8.
This humidifying system 100 for a fuel cell mainly comprises: a fuel cell 101 having the anode 101a to which hydrogen in a fuel gas is supplied and the cathode 101c to which oxygen in the air is supplied as an oxidant gas so that hydrogen and oxygen are reacted to generate electricity; two humidifiers 102, 103 humidifying gases respectively supplied to the anode 101a and the cathode 101c of the fuel cell 101 by moisture-exchanging with cathode exhaust gas discharged from the cathode 101c of the fuel cell 101; an ejector 104 supplying a fuel gas to the anode 101a in a circulating manner; and a supercharger (S/C) 105 supplying air as an oxidant gas to the cathode 101c. Herein, the fuel cell 101 is considered as a constituent element of the humidifying system 100.
Manner of operation of this humidifying system 100 will be described.
A fuel gas containing no moisture or a little moisture is adjusted by a regulator 106 to a constant pressure and supplied to an ejector 104. The fuel gas is then fed to a humidifier 102 after passing through the ejector 104.
The fuel gas (low humidity gas) supplied to the humidifier 102 is humidified by cathode exhaust gas (high humidity gas) discharged from the cathode 101c of the fuel cell 101 while passing through a humidifying module within the humidifier 102, and then supplied to the anode 101a of the fuel cell 101. Hydrogen contained in the fuel gas supplied to the anode 101a reacts with oxygen in the air that is supplied from the S/C (supercharger) 105 to the fuel cell 101, and generates electricity. Remaining fuel gas that has been unreacted at the fuel cell 101 is supplied to the subsequent steps (e.g., catalyst combustor) as anode exhaust gas. Part of the anode exhaust gas is sucked by the ejector 104 and recirculates as a fuel gas.
Meanwhile, air that is a low humidity gas in the air is sucked and pressurized by the S/C (supercharger) 105, and is supplied to the humidifier 103.
The air (low humidity gas) supplied to the humidifier 103 is humidified by cathode exhaust gas (high humidity gas) discharged from the humidifier 102 while passing through a humidifying module within the humidifier 103, and then supplied to the cathode 101c of the fuel cell 101. Remaining air that has been unreacted at the fuel cell 101 with hydrogen in the fuel gas is firstly supplied to the humidifier 102 as cathode exhaust gas which is a high humidity gas. The cathode exhaust gas supplied to the humidifier 102 gives moisture to the fuel gas supplied from the ejector 104 to the humidifier 102 while passing through the humidifying module within the humidifier 102, and is discharged from the humidifier 102. Subsequently, the cathode exhaust gas discharged from the humidifier 102 is supplied to the humidifier 103, and gives moisture to air supplied from the S/C (supercharger) 105 to the humidifier 103 while passing through the humidifying module within the humidifier 103. Cathode exhaust gas discharged from the humidifier 103 is supplied to the subsequent steps (e.g., catalyst combustor).
However, such a conventional humidifying system for a fuel cell has the following drawbacks.
(1) A fuel gas supplied to the fuel cell 101 is required to have a constant relative humidity. However, heat of reaction increases proportionally with increasing output of the fuel cell 101 because the chemical reaction at the fuel cell is exothermic reaction, and the temperature of exhaust gas discharged from the fuel cell 101 inevitably increases. For this reason, in the humidifier 102 where a fuel gas supplied to the fuel cell 101 is humidified, a fuel gas is humidified by high temperature cathode exhaust gas containing supersaturated moisture (having high partial pressure of water vapor), and as shown in FIG. 9, the fuel gas is over-humidified (the dew point becomes higher) relative to the target dew point range required for the fuel cell 101. As a result, if cathode exhaust gas continues to humidify the fuel gas, so-called flooding may arise, that is, water pools in gaps formed between solid polymer electrolyte membranes, which are stacked in the form of a fuel cell stack, and clogs gas flow passages.
(2) Meanwhile, in the humidifier 103 where air supplied to the fuel cell is humidified, since a fuel gas is over-humidified in the humidifier 102 by cathode exhaust gas, the amount of moisture for humidifying air that is an oxidant gas supplied to the fuel cell 101 is liable to be insufficient, and as shown in FIG. 9, the dew point of air lowers relative to the target dew point range required for the fuel cell 101. As a result, the solid polymer electrolyte membrane of the fuel cell 101 gets dry, preventing stable generation of electricity.
The inventors attempt to vary the length and the number of the hollow fiber membranes in the humidifier 102 for the purpose of preventing a fuel gas from being over-humidified. However, in the graph of FIG. 9 illustrating the relation between the output of the fuel cell 101 and the dew point, the dew point of a fuel and the dew point of air merely shifted vertically, and it was impossible to obtain a horizontal slope for the lines. Furthermore, when the fuel cell 101 was operated at a high output, another drawback arose in that the amount of humidification was insufficient.
In view of the above, the present invention provides a humidifying system for a fuel cell which achieves the optimum balance between the humidification of a fuel gas supplied to the fuel cell and the humidification of an oxidant gas supplied to the fuel cell even at a high output of the fuel cell 101.